MOSFET RF POWER FOR ACCELERATOR APPLICATIONS1

J.-M. Godefroy, J. Polian, F. Ribeiro, T. Ruan

LURE, Centre Universitaire Paris-Sud, Bât. 209A, 91898 Orsay Cedex, France

Abstract 5 V R5 C6 D1

F1 F3 R1 P1 L1 R7 An inexpensive reliable 500 MHz 1 kW CW R3 Q1 T1 C1 T3 T5 C11 T7 MOSFET solid state has been developed INPUT OUTPUT CON1 B1 C3 C4 C5 C8 C9 C10 B2 CON2 for accelerator applications. It presents the following 44 dBm 150 W max. T2 T4 T8 CIR1 C2 R4 T6 C12 advantages : Each 150 W module is protected by a 4.5 A P2 L2 R2 F5 surface mount circulator and is equipped with input and F2 F4 28 V F6 D2 output trimmer capacitors to assure perfect matching, 4.5 A equal and high efficiency ; Only two pieces of 5 V R6 C7 simple 8-way impedance transformer are used for RF power distribution and combination. Based on this Figure 1. Schematic diagram of 150 W module. modular amplifier, low cost, very high power solid state may easily be developed. A surface mount circulator CIR1 with a termination R7 is used for protecting the MOSFET, it improves amplifier stability and simplifies the power combiner. 1. INTRODUCTION R1 and R2 are used for improving the stability at low frequency (about 180 MHz). A 1 kW amplifier is needed to drive the TH563 In order to obtain a compromise between efficiency tetrode in the new 500 MHz fifth harmonic RF system for and linearity, Class-AB operation is used. The 2 × 160 the storage ring Super-ACO [1]. mA quiescent drain-source currents are set by adjusting New generation solid-state devices, i.e. silicon RF the potentiometers P1 and P2. power are used. They have a number of advantages over their bipolar counterparts : high power 3. MEASUREMENT CIRCUIT gain ; no thermal runaway ; low noise figure; low feedback capacitance; easy biasing ; gold metallization In order to obtain optimum operation conditions and ensuring excellent reliability ; etc. identical gain for all amplifier modules, the measurement For a storage ring, the RF frequency is fixed, circuit, shown in Figure 2, is used. allowing the design and development of a special-purpose amplifier for optimum performance : higher efficiency, NETWORK ANALYSER POWER METER SPECTRUM perfect matching, lower harmonics [2]. ANALYSER

2. MODULE TRM1 10 dB 20 dB ATTENUATOR ATTENUATOR A1 A2 The principal basic unit of the 1 kW amplifier is a A C TRM4 500 MHz 150 W amplifier module. Its schematic diagram GENERATOR COUPLER 1 B COUPLER 2 D is shown in Figure 1. 20 dB 20 dB In Figure 1, Q1 is a BLF548 power MOSFET made CIRCULATOR TRM2 TRM3 by Philips Semiconductors. It has a push-pull Figure 2. Measurement block diagram. configuration. Therefore on the input and output circuits, the Baluns B1 and B2 are necessary, both consist of a In Figure 2, A2 is one of the 150 W amplifier section of 50 Ÿ VHPLULJLG FRD[LDO FDEOH :H WDNH WKHLU modules that we want to measure. length equal to 67 mm. Therefore the striplines T1, T2, The Generator (R/S SMT03) delivers an RF signal, T3 and T4 act as the 3rd harmonic filters. which is amplified by a 25 W amplifier A1 and passes T3 and T4 are striplines. They are used for input and through the Circulator and Directional Coupler 1 to drive output matching. the 150 W amplifier module. C3, C4, C9 and C10 are trimmer capacitors. They Because all instrument inputs are low level, we must are used for adjusting input and output matching. insert the Directional Couplers 1 and 2.

1 Work supported by CNRS, CEA, MENESR.

1811 The network analyser (HP 58753D) is set to A/B The value of output power is measured behind a mode and its internal generator is switched off. When circulator, whose insertion loss is about 0.3 dB. testing the power gain, we compare the Signal C with Nevertheless the efficiency is still equal to 60 - 65 % at Signal A from the Directional Couplers 2 and 1. When 150 W output power. testing S11, we compare Signal B with Signal A from the Directional Coupler 1. 4.3. Input Matching TRM4 is a 50 Ÿ kW load resistor. The Power Meter (R/S NRVD) with the Sensors Input impedance depends on output power. The S11 (NRV - Z5) is used for RF power measurements. parameter is shown in Figure 5.

0 4. CHARACTERISTICS OF MODULE S11 (dB) 4.1. Gain and Phase Shift -20

The RF power gain as a function of output power is -40 shown in Figure 3.

14 -60

G (dB) 12

-80 10 0 20 40 60 80 100 120 140 160 180 200

Pout (W) 8 Figure 5. S11 versus output power. 6

4 From Figure 5, we see that between 137 - 152 W, S11 is less than -40 dB. 2

0 0 20 40 60 80 100 120 140 160 180 200 4.4. Harmonics Pout (W) Figure 3. Gain versus output power. We measured the harmonics at 50, 100, 150 W output power with a spectrum analyser (Tektronix 497P). Its phase shift is a function of output power also. The maximum harmonic content occurs at 150 W output According to measurement, between 125 W and 150 W power, it is the 2nd harmonic and is equal to -42 dB. For output, the phase difference is only 1°. But between 25 W a push-pull circuit, normally the maximum harmonic is and 125 W output, it becomes -0.136°/W. the 3rd harmonic, however our circuit contains the equivalent of 3rd harmonic filters (T7 and T8 in 4.2. Efficiency Figure 1), which reduce this harmonic to -68 dB.

The efficiency as a function of output power is 4.5. Frequency Characteristics shown in Figure 4. In order to study and understand the RF power 0,8 MOSFET amplifier, we measured all frequency characteristics at different power levels.

EFFICIENCY 0,6 At the 499.396 MHz operation frequency and 150 W output power, we can adjust the trimmer capacitors to maximize efficiency and minimize S11. 0,4 But the maximum gain S21 does not always occur at operation frequency, it is dependent on the output level.

0,2 At very low level, the maximum S21 occurs at 507 MHz, at 150 W output it occurs at 489 MHz. The phase shift characteristic is linear around the 0,0 0 20 40 60 80 100 120 140 160 180 200 operation frequency. The group delay is about 14 ns. Pout (W) At 499.396 MHz and 150 W output power, the input Figure 4. Efficiency versus output power. matching S11 is less than -50 dB, and within 6 MHz, it is less than -20 dB.

1812 But the minimum S11 does not always occur at There is no circulator in the 25 W preamplifier U1, operation frequency. It is dependent on output power. At so the coaxial cable length, between U1 and U2, is low level, the minimum S11 occurs at 513 MHz and is important for linearity. Taking the optimum length can equal to -17 dB. improve the total linearity of the 1 kW amplifier by 4 dB. In Figure 3, within 150 W, the maximum difference in 4.6. Stability gain is 3 dB. There are two stages of 150 W module in the 1 kW amplifier, so the maximum difference in total gain For an RF power amplifier, stability is very could be 6 dB. But due to this compensation, the important. For accelerator applications, generally maximum difference in total gain is only 2 dB. Unconditional Stability is preferred. At 1 kW output power, the total gain is 61 dB and We measured the S parameters from 100 MHz to the total efficiency is 50 %. 2 100 MHz at low power level with the network analyser. Then we calculated the stability condition for these 6. CONCLUSION frequencies. The result is that the stability of our module is Unconditional with 5.6 dB gain protection, because For accelerator applications, it is preferable to there is a circulator with 18 dB isolation in our module. develop special-purpose RF power amplifiers, because all Without it, the stability would be Conditional. commercial amplifiers or modules are designed for broadband applications, so their efficiency is lower, 5. 1 kW AMPLIFIER harmonic contents are higher and matching is less good. A comparison of specifications between our module and a The 500 MHz MOSFET 1 kW amplifier block commercial class-AB 150 W MOSFET amplifier module diagram is shown in Figure 6. is shown in Table 1.

U3 U4 U5 Table 1. Comparison of specifications.

U6 Our Module Commercial U7 Efficiency 60 % 30 %

U1 U2 U8 Input VSWR 1.02 2 1 kW U9 Load VSWR Infinite 3 OUTPUT 25 W 150 W U10 IEC50-22 2nd harmonic -42 dB -20 dB MODULE MODULE U11 3rd harmonic -68 dB -12 dB U12 Stability Unconditional Conditional

SPLITTER 8X150 W COMBINER MODULES Since March 1997, this 500 MHz 1 kW MOSFET amplifier has been functioning without incident. Figure 6. 1 kW amplifier block diagram. In 1997, a 100 MHz 1.5 kW MOSFET amplifier was also developed to replace two 4CW800B tetrodes in In Figure 6, U1 is a commercial 25 W module with parallel based pre-amplifier for driving a 4CW50000E 39 dB of gain. It works in Class-A operation. tetrode. It also works very well. U2 and U5 - U12 are 150 W modules, which are A low cost 352 MHz 30 kW CW MOSFET water cooled. U3 is an 8-way RF power splitter. U4 is an amplifier for the new project SOLEIL is currently under 8-way RF power combiner. They were made in our development. Its cost could be lower than the cost of a laboratory. tetrode amplifier. At 150 W output power, among the nine 150 W modules, the maximum difference in gain is 0.045 dB and ACKNOWLEDGEMENTS the maximum difference in phase is 7.35°. For the RF power splitter, S11 is -41 dB. Between We are greatly indebted Mr A. Muller and the 8 outputs, the maximum difference in S21 is 0.04 dB Mr E. Dauer for manufacturing and assembling this 1 kW and 0.74°. amplifier; Mr G. Flynn, for fruitful discussions. For the RF power combiner, S11 is -35 dB. Between the 8 inputs, the maximum difference in S21 is 0.04 dB and 0.53°. REFERENCES For the RF coaxial cables, which are between the splitter and 150 W modules, as well as combiner, the [1] G. Flynn et. al., “A New 500 MHz Fifth Harmonic maximum S11 is -30.3 dB and the maximum difference in RF System for Super-ACO”, this conference. S21 is 0.01 dB and 2.94°. [2] T. Ruan et. al., “Amplificateur 500 MHz 1 kW MOSFET”, Super-ACO/98-03.

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